A method of initial access in a wireless communication system according to related art will be described with reference to FIGS. 1 and 2. FIG. 1 illustrates a method of initial access in a wireless communication system according to related art. FIG. 2 illustrates ranging process between a mobile station and a base station in a wireless communication system according to related art.
As illustrated in FIG. 1, to perform initial access, a mobile station first scans downlink channel (FIG. 1, S101; FIG. 2, S210) and searches base stations which the mobile station is accessible to. At this time, the mobile station scans frequency of neighbor base stations one by one because having no information about network configuration.
And, after the mobile station acquires system information about downlink and uplink from the searched base station and completes system setting (FIG. 1, S102 to S104; FIG. 2, S220), the mobile station performs ranging process with the searched base station (FIG. 1, S105, S106; FIG. 2, S230, S240). The mobile station selects a CDMA ranging code and performs contention based ranging process with the base station and establishes synch (FIG. 1, S105, S106; FIG. 2, S250, S260, S270, S280).
Until synch is established, the base station informs the mobile station of values of parameters which must be corrected through ranging response (RNG-RSP) messages. While values of parameters are correcting, status of RNG-RSP messages is set to continue. And after correcting values of parameters is completed, status of RNG-RSP messages is set to success (FIG. 2, S240, S250, S260).
After synch is established, the mobile station registers in a wideband network and is served (FIG. 1, S107 to S120; FIG. 2, S290).
Delay of a method of initial access according to related art is not that long in a previous wireless communication system because number of base stations is not that large and type of base stations is one. But, as new types of base stations like femto cell base stations and relay base stations appear recently, delay is too long if a method of initial access according to related art is applied to a wireless communication system including new types of base stations. A method of initial access which is proper for new types of base stations becomes necessary.
First, there are transparent and non-transparent types in relay base stations, and it is determined which base station a mobile station transmits control signals to by the type of a relay base station.
A transparent relay base station exchanges user plane information with a mobile station and control signals are exchanged between a mobile station and a macro base station. that is, it is a centralized BS-oriented network.
A non-transparent relay base station exchanges user plane information and control signals with a mobile station. And it can be a centralized BS-oriented network or a distributed BS-oriented network.
A femto cell base station is a mini version of a macro base station and performs almost functions of a macro base station. A femto cell base station can be established in coverage of a macro base station or in area which is not covered by a macro base station. A femto cell base station has a network which operates independently. And a femto cell base station will be established in indoor or urban area far more than a relay base station. Thus, femto cell base stations are not included in a neighbor base station list which a base station transmits to a mobile station because overhead is too large.
Because there are too many femto cell base stations around a mobile station, delay becomes too long if a mobile station scans all of neighbor base stations when searching downlink channel like a method of initial access according to related art.
There are a select first and reject later method and a select right method in methods of initial access in a wireless communication system including femto cell base stations.
In a select first and reject later method, a mobile station scans downlink channel and tries ranging to a femto cell base station which is detected first. Then the femto cell base station checks whether the mobile station is accessible to itself and authenticated. If the mobile station is not accessible to itself or authenticated, the femto cell base station rejects the mobile station. The rejected mobile station scans downlink again and searches another femto cell base station and tries ranging to the searched femto cell base station. A select first and reject later method has a problem that delay of initial access is large because a mobile station repeats ranging until finding a femto cell base station which a mobile station is accessible.
In a select right method, a mobile station finds out a femto cell base station which a mobile station is accessible at once with a specific method. However, the specific method must be defined in a select right method.